Ink jet printers having one or more ink jet heads for projecting drops of ink onto paper or other printing medium to generate graphic images and text have become increasingly popular. To form color images, ink jet printers with multiple ink jet printing heads are used, with each head being supplied with ink of a different color. These colored inks are then applied, either alone or in combination, to the printing medium to make a finished color print. Typically, all of the colors needed to make the print are produced from combinations of cyan, magenta and yellow inks. In addition, black ink may be utilized for printing textual material or for producing true four-color prints.
In a common arrangement, the print medium is attached to a rotating drum, with the ink jet heads being mounted on a traveling carriage that traverses the drum axially. As the heads scan paths over the printing medium, ink drops are projected from a minute external orifice in each head to the medium so as to form an image on the medium. A suitable control system synchronizes the generation of ink drops with the rotating drum.
In one basic type of ink jet head, ink drops are produced on demand. An exemplary drop-on-demand ink jet head is illustrated in U.S. Pat. No. 4,106,032 of Miura et al. The Miura et al. ink jet head has a two compartment ink chamber comprised of an inner horn compartment and an outer ink compartment which communicate with one another through a connecting channel of restricted cross section. Ink is delivered to the outer ink compartment of the ink jet head. Whenever a drop of ink is needed, an electric pulse is applied to a piezoelectric crystal, causing the crystal to constrict. As a result, because the crystal is in intimate mechanical contact with ink in the horn compartment, a pressure wave is transmitted through the ink chamber. In response to this pressure wave, ink flows from the outer ink compartment and through an ink orifice passageway in an ink chamber wall and forms an ink drop at an internal ink drop-forming orifice outlet located at the outer surface of the ink chamber wall. The ink drop passes from the ink drop-forming orifice outlet and through an air chamber toward a main external orifice of the ink jet head. This latter orifice is aligned with both the internal orifice and the connecting channel and also leads to the printing medium. Air under pressure is delivered to the air chamber and entrains the drop of ink in a generally coaxial air stream as the ink drop travels through the air chamber. This air stream increases the speed of the drops toward, and the accuracy of applying the drops to, the print medium.
Such ink jet heads, as well as ink jet heads of the non-air assisted type, can easily become clogged with contaminants. Also, air bubbles within these ink jet heads can interfere with or block their operation. There are many potential sources of such air bubbles and contaminants. For example, air bubbles may be introduced into the ink inside the ink chamber through the ink orifice passageway. Also, air bubbles may be generated in the ink as temperature or pressure changes. For example, during transportation or shipment of an ink jet head at high altitudes by airplane or operation of such an ink jet head at high altitude locations.
Various prior art devices have been developed for removing air bubbles and contaminants from ink jet heads. For example, U.S. Pat.No. 4,466,005 of Yoshimura discloses an air bubble removing system for an ink jet head which operates by applying purging drive signals of various fixed frequencies and various voltages to a piezoelectric crystal utilized to drive the ink jet head. These signals break up air bubbles to facilitate their discharge from the jet head. In the example described in this Patent, the purging drive signals are one kilohertz, one hundred and twenty-five hertz and four hertz. In addition, an ink jet printer commercially available from Tektronix, Inc. of Beaverton, Oreg., model number 4692, also employs this technique of applying stepped frequency purging signals. In the 4692 apparatus, purging signals of fifteen, twenty and thirty kilohertz are applied to a piezoelectric crystal to assist in removal of air bubbles from the ink chambers of ink jet heads. In both the Tektronix 4692 ink jet printer and apparently in the Yoshimura system, contaminants are discharged through a restricted orifice. During discharge, these contaminants can become lodged in the orifice and disable the ink jet head. It is known that air bubbles vibrate when subject to pressure pulses at the resonant frequency of the air bubbles. It is also known that such oscillations assist in separating the air bubbles from the wall of a chamber. For example, this is described in an article entitled "Acoustic Methods Remove Bubbles From Liquids," NASA Tech Brief, Vol. 7, No. 2, Item No. 53, published in 1982 and also in a Jet Propulsion Laboratory Invention Report, NPO-15334/5046, published in July 1983. In the NASA Tech Brief, a disclosure which does not mention ink jet heads, a method of removing bubbles from a liquid bath is described. In this method, the bath is swept with frequency signals, generated by a voltage controlled oscillator, over a range of from 0.5 kilohertz to forty kilohertz.
In another prior art approach, U.S. Pat. No. 4,533,569 of Bangs discloses an ink jet head in which an interior surface of a glass ink jet nozzle is cleaned with a chemical solution to minimize air bubble formation and to facilitate purging of air bubbles from the nozzle. Also U.S. Pat. No. 4,518,974 of Isayama discloses a system for removing air bubbles in which an air-ink boundary is drawn temporarily within a nozzle chamber and toward an ink supply side of the chamber. When this occurs, a transfer of air within the nozzle to the atmosphere is permitted. As still another approach, U.S. Pat. No. 4,518,973 of Tazaki discloses a suction recovery apparatus which applies a negative pressure to a nozzle orifice outlet for removal of air bubbles and contaminants from the nozzle. These approaches all suffer from a number of limitations.
In addition to the problem of purging bubbles and cleaning contaminants from ink jet heads during operation, it is difficult to initially fill ink jet heads with ink without introducing air bubbles into the ink within the ink jet head. In a common approach, such as utilized with the Tektronix 4692 ink jet printer, ink jet heads are initially filled as follows. First, a vacuum is drawn on the ink chamber of the ink jet head in order to remove air from the ink chamber. Then, the ink chamber is filled with water which is eventually replaced with ink. Typically, these ink jet heads have two ink chamber compartments, such as in U.S. Pat. No. 4,106,032 of Muir et al. In addition, these ink jet heads are provid a port leading to the horn chamber for use in filling the ink jet head. Following filling, a screw is utilized to close this port. This ink jet head filling is performed while the ink jet head is removed from an ink jet printer and typically is extremely time consuming. It should be noted that it is extremely difficult to remove air bubbles which happen to be present in the horn chamber during such a filling operation. As another example of this approach, FIGS. 13 and 14 of U.S. Pat. No. 4,380,018 of Andoh et al. discloses a two compartment ink chamber with an ink filling port. In the FIG. 13 form, a passage is provided between an outer ink chamber and an inner horn chamber. The ink filling port communicates with this passageway. A screw is utilized to plug this port following filling. During normal operation of this ink jet head, ink is supplied to the outer compartment. The FIG. 14 embodiment eliminates the passageway between the outer ink compartment and horn compartment. However, like the FIG. 13 form, the ink filling port is plugged during normal operation of the ink jet head and ink is supplied to the outer ink compartment.
Also, U.S. Pat. No. 4,312,010 of Doring discloses a non-air assisted ink jet head having a flat conical single compartment fluid chamber. Because of the shape of this chamber, during filling with ink, an air bubble is enclosed by the ink and forced out through an orifice at the apex of the conical ink chamber.
Although these various approaches for filling ink jet heads and for purging air bubbles and contaminants from the ink jet heads are known, a need exists for an improved method and apparatus for this purpose.